This application relates to polymeric gels, in particular crosslinked flame retardant thermoplastic elastomer gels.
In today's modern electrical and electronic devices, as well as in other uses such as fiber optic connections, sealants are often used for insulation, for protection against water, corrosion and environmental degradation, optical index matching, and thermal management. Prior to now, a number of sealants and gels have been known, however, none of these previous sealants and gels have the same components and offer the same combination of properties as the compositions described and claimed herein.
It is known that sealants have been designed specifically to self-extinguish upon exposure to flame. These flame retardant sealants may self-extinguish using the mechanism known as intumescence. When an intumescent flame retardant material is introduced to flame, some or all of the material will quickly form a char barrier. The char barrier is typically foamed away from itself. By foaming away, the char barrier blocks oxygen and the ambient flame from reaching the fuel source. As a result, the intumescent flame retardant material may self-extinguish.
Sealants have been made that incorporate halogen-based additives. Halogen-based additives can provide relative protection from fire hazards. Some halogen containing compositions suffer from inadequacies and can generate, upon combustion, corrosive hydrogen halides. Moreover, some halogenated flame retardants may be considered hazardous and are not Reduction of Hazardous Substances (“RoHS”) compliant.
Sealants have been made that incorporate metal hydroxides as flame retardants. However, compositions generally require large amounts of the metal hydroxides to be effective for imparting flame retardancy. Metal hydroxides are generally not suitable for developing soft flame retardant thermoplastic elastomer compositions as they can increase specific gravity and can undesirably increase the hardness of compositions.
As technology progresses, sealants will be subjected to increasingly higher temperature environments and more demanding performance requirements. There has been, and there presently exists, a need for high performance sealants to meet these demands.
Gels, for example, have been used as sealants with relative success in certain applications due to their unique properties. Gels may have a lower hardness than rubber and can seal and conform under adequate compression. Gels may also be more elastic than mastics. Other advantages of gels are known in the art. For example, gels, when used as sealants, may be removed and re-entered more easily due to elastic recovery of the gel. For further example, relatively little force is required to change the shape of a soft gel sealant.
Solid particulates have been added to alter a gel's properties. However, one of the problems with flame retarding a soft gel is that the addition of solid particulate fillers leads to hardening and produces a gel with poor sealing properties. Other disadvantages of gels are known in the art.
One class of gels used as a sealant is thermoplastic elastomer gels (TPEGs). Certain TPEGs have advantages over other classes of gels such as silicone gels, polyurethane gels, and polybutadiene gels. For example, silicone gels may have a higher cost compared to TPEGs, a silicone gel's dielectric breakdown voltage may be adversely affected by humidity, and low surface energy silicone oils can leak or evaporate out of the gel and spread over electrical contact points leading to problematic insulation barriers.
Problems with polyurethane and polybutadiene gels include, for example, hydrolytic instability of the crosslinked network; and degradation and hardening with aging. In addition, environmental concerns regarding certain non-TPEGs have led to an increased interest in developing gels with enhanced safety profiles while achieving sufficient or enhanced properties.
TPEGs have provided many years of reliable in-field performance for applications requiring a low maximum service temperature of approximately 70° C. TPEGs have been made that comprise a styrene ethylene/butylene styrene (“SEBS”) triblock copolymer swollen with a mineral oil softener. While the thermoplastic nature of these gels allows for easy production, it limits the upper service temperature due to creep and flow as in-field ambient temperatures approach the styrene glass transition. Research has been aimed at increasing the upper service temperature of these gels through chemically crosslinking the gel network in order to form a thermoset gel structure. For example, oil-swelled acid/anhydride modified maleic anhydride SEBS gels have been covalently crosslinked using small molecule crosslinkers like di- and triamines, see European Publication No. EP 0879832A1 (Wang et al.), published Nov. 25, 1998, as well as with some metal salts, D. J. St. Clair, “Temp Service,” Adhesives Age, pp. 31-40, September 2001. Crosslinked polymers are known to increase thermal stability, toughness, and chemical resistance compared to their base, or uncrosslinked polymers. However, crosslinked polymers are also known to often be intractable, making them difficult to reprocess or recycle.
In addition, safety is a growing concern for sealants and gels. Governments and producers of sealants and gels have taken and will continue to take steps to reduce the use of materials that may be hazardous to people's health. In particular, efforts have been taken to reduce and eliminate halogenated flame retardants (e.g., polybrominated biphemyls (PBB), poly brominated diphenyl ether (PBDE)) in sealants and gels. Other problems and attempts to address them are known in the art.
U.S. Pat. No. 6,207,752 to Abraham et al. relates to low oil swell carboxylated nitrile rubber-thermoplastic polyurethane vulcanizate compositions. The nitrile rubbers of Abraham contain pendant carboxyl groups that can be crosslinked. The patentees report unexpectedly discovering that a processing aid can improve the processability of the compositions. The patent lists a number of processing aids including maleated polyethylene, maleated styrene-ethylene-butene-styrene-block copolymers and maleated styrene-butadiene-styrene-block copolymers, and maleated ethylene-propylene rubber.
U.S. Pat. No. 6,756,440 to Hase et al. relates to a fire resistant resin composition, a method of making the resin composition and an electrical wire comprising the composition. The composition has a halogen-free propylene resin containing propylene as a monomer component, a halogen-free styrene-based thermoplastic elastomeric resin modified with an unsaturated carboxylic acid or a derivative of such an acid, and a fire resistant metal hydroxide.
U.S. Published Patent Application No. 2002/0065356 to Crevecoeur et al. relates to flame retardant polymers with a condensation polymer, a halogen-containing styrene polymer, a polymer derived from aromatic vinyl monomer, and elastomeric polymer segments. The polymers derived from aromatic vinyl monomers may be crosslinked.
International Publication No. WO 98/40436 to Perkins et al. relates to flame retardant gels of oil-extended triblock copolymers. The gel compositions may contain ammonium polyphosphate and synergists such as organic amine phosphates and melamine phosphate. The compositions may contain polyols such as polyerythritol.